1. Field of the Invention
This invention relates to a device for detecting an incident position of radiation beam on a predetermined surface.
2. Description of Prior Arts
Speaking, for example, of a field of a detector such as a range finder to measure a distance to an object or a focus detecting device to find out an in-focus condition of an image forming optical system to an object, there have already been known the so-called "active type distance measuring device" or "focus detecting device", in which a distance to an object is measured, or the in-focus, condition of the image forming optical system to the target object is detected, by providing a radiation projecting means on the side of the measuring device, and projecting radiation beam such as infrared or near infrared ray toward the object, at which time variations in the incident position of the reflected radiation beam from the object on a predetermined surface is utilized for distance measurement on the target object.
U.S. patent application Ser. No. 43,250, now U.S. Pat. No. 4,357,085, of the same assignee as that of the present application discloses an example, in which a self-scanning type linear image sensing means such as CCD, BBD, MOS image sensor, etc. is adopted as the means for detecting the reflected radiation beam in such active type range finding device and the focus detecting device.
The abovementioned earlier application discloses as one embodiment thereof a device for detecting an incident position of reflected radiation beam, as one of the expedients for detecting the incident position of the reflected radiation beam in utilization of a time-sequential output of a sensor array device having a plurality of linearly arranged radiation sensing elements as the self-scanning type linear image sensing means, wherein a slice level is determined at a certain definite ratio on the basis of a peak value of the time-sequential output, based on which slice level another time-sequential output from the sensor array device to be obtained at the subsequent reading is sliced, and then a position of the sliced signal portion in the time-sequential output signal is detected. However, even with this embodimental device in the earlier application, there still remain some points to be improved. For example, this embodimental device is to judge the incident position of the reflected radiation beam by detecting a peak value of an envelope of the time sequential signal which has first been read out of the sensor array device, then setting and maintaining a slice level to be determined from this peak value at a predetermined ratio, thereafter slicing the signal envelope which has second been read out based on the determined slice level, and finally digitally determining whether the signal envelope portion which is about to be sliced is at the center of the signal envelope as a whole, or is deviated from the center to either direction of left or right. In such method of determination, the outputs from the sensor array device should be continuously read out for two times with a view to detecting the incident position of the reflected radiation beam with the consequence that a long period of time is required for the determination. Further, since the detecting limit of the signal depends on the slice level, it becomes difficult by slicing to distinguish the signal envelope in the upper and lower portions, when the peak value of the output signal becomes lowered due to the degree of convergence of the radiation beam, or due to an object being at a far distance, or an object having a low reflective power with the consequence that errors tend to occur in determining the incident position of the reflected radiation beam, and other points of problem. In view of such difficulties and inconveniences, there still remains rooms for improvements in the embodimental device of the earlier application.
Besides the above problem points, in order to attain accurate and highly precise detection of the incident position of the radiation beam, a gain control with respect to the output from the sensor array device and/or an appropriate gain control in the subsequent signal processing circuit system are essential. In this point, too, there still remains room for improvement.
Furthermore, as disclosed in the afore-described earlier U.S. patent application Ser. No. 43,250, when a signal radiation beam such as infrared or near infrared ray is projected from the device to an object, and a reflected signal radiation beam from the object is to be detected, there should essentially be made a clear distinction, for the sake of the signal processing, between radiation due to external light, etc. existing in the surrounding atmosphere and the signal radiation beam emitted from the device, in other words, there should inevitably be made a contrivance for effective elimination of the noise signal component caused by the radiation due to external light, etc. In particular, as disclosed in this earlier filed application, when the signal radiation beam is projected toward a target object to be focussed through an object lens, and a reflected signal radiation beam from the target object is to be detected through the object lens by the sensor array device (i.e., in the case of the so-called TTL (Through The Lens) type, active focus detecting device), the intensity distribution of harmful radiation due to external light, etc. existing in the surrounding atmosphere on the radiation receiving surface of the sensor array device becomes non-uniform. In order therefore to remove the noise signal component which is derived from such harmful radiation and is unavoidably included in the output from the sensor array device, a further and particular contrivance is required.